In a gas bag having a discharge opening it is desirable for the discharge opening to remain closed, as far as possible, during the inflation process, so that the quantity of gas necessary to completely fill the gas bag can be kept low. It is desirable in addition to match the hardness of the gas bag to the respective circumstances. As is known in the art, this can be achieved by altering the cross-section of the discharge opening depending on the internal pressure of the gas bag. In this way, if the unfolding gas bag comes into contact prematurely with the vehicle occupant or with an obstacle, a discharging of gas is permitted during the unfolding phase. Furthermore, through the pressure-dependent opening of the discharge opening, the stress of the vehicle occupant, who is plunging into the gas bag, can be influenced in a regulating manner, in particular with regard to his body weight.
British Patent Application GB 2 302 845 A shows a gas bag having a discharge opening which is closed by a covering sewn on the gas bag. When the vehicle occupant plunges into the gas bag, the covering may be detached by a tension means fastened to the covering, in order to open the discharge opening.
From European Patent Application EP 1 393 989 A1 it is likewise known to secure a discharge opening by a flap which is fastened to the gas bag wall. The flap is only detached when a particular internal pressure is exceeded. The discharge opening, which is formed by several cuts in the gas bag wall, is smaller than the discharge cross-section which results after the detachment of the flap.
A gas bag for a vehicle occupant restraint arrangement having a discharge opening which is formed by an overlapping of openings in several layers of fabric lying one over another is known from German Utility Model DE 296 09 703 U1. The layers of fabric are moveable relative to each other depending on the internal pressure of the gas bag.
The outer fabric layer has a gathered region which is secured by a tear seam. This seam tears open or not, depending on how much gas is provided by a multiple-stage gas generator for filling the gas bag. On tearing open, the outer fabric layer moves with its opening relative to the opening of the inner fabric layer, whereby the effective discharge cross-section is altered.
It is an object of the invention to provide a gas bag which makes possible an automatic and very variable adaptation of the hardness of the gas bag without such additional measures.